Assignment: group override sort order is not working

Found during review the MDLQA-16555.

When we were trying to change the order of the group overrides, an error will be shown.

Steps to replicate:

1. Create a course.
2. Create 2 groups called Group 1 and Group 2.
3. Create an Assignment activity.
4. Create 2 group overrides for Group 1 and Group 2 in the Assignment activity.
5. Try to change the order of the group overrides.